Re: [PATCH] x86/asm/64: do not clear high 32 bits of syscall number when CONFIG_X86_X32=y

[Andy Lutomirski]

On Tue, Sep 12, 2017 at 3:57 PM, Dmitry V. Levin <ldv@xxxxxxxxxxxx> wrote:
> Before this change, CONFIG_X86_X32=y fastpath behaviour was different
> from slowpath:
>
> $ gcc -xc -Wall -O2 - <<'EOF'
> #include <unistd.h>
> #include <asm/unistd.h>
> int main(void) {
>         unsigned long nr = ~0xffffffffUL | __NR_exit;
>         return !!syscall(nr, 42, 1, 2, 3, 4, 5);
> }
> EOF
> $ ./a.out; echo \$?=$?
> $?=42
> $ strace -enone ./a.out
> syscall_18446744069414584380(0x2a, 0x1, 0x2, 0x3, 0x4, 0x5) = -1 (errno 38)
> +++ exited with 1 +++
>
> This change syncs CONFIG_X86_X32=y fastpath behaviour with the case
> when CONFIG_X86_X32 is not enabled.

Do you see real brokenness here, or is it just weird?

>
> Fixes: fca460f95e92 ("x32: Handle the x32 system call flag")
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Dmitry V. Levin <ldv@xxxxxxxxxxxx>
> ---
>  arch/x86/entry/entry_64.S | 8 +++-----
>  1 file changed, 3 insertions(+), 5 deletions(-)
>
> diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
> index 4916725..3bab6af 100644
> --- a/arch/x86/entry/entry_64.S
> +++ b/arch/x86/entry/entry_64.S
> @@ -185,12 +185,10 @@ entry_SYSCALL_64_fastpath:
>          */
>         TRACE_IRQS_ON
>         ENABLE_INTERRUPTS(CLBR_NONE)
> -#if __SYSCALL_MASK == ~0
> -       cmpq    $__NR_syscall_max, %rax
> -#else
> -       andl    $__SYSCALL_MASK, %eax
> -       cmpl    $__NR_syscall_max, %eax
> +#if __SYSCALL_MASK != ~0
> +       andq    $__SYSCALL_MASK, %rax
>  #endif
> +       cmpq    $__NR_syscall_max, %rax

I don't know much about x32 userspace, but there's an argument that
the high bits *should* be masked off if the x32 bit is set.  Of
course, that's slower, but it could be done without performance loss,
I think.

--Andy